My site has a similar problem.
This post is the most recent one with double titles.
https://thegpsgeek.com/blog/archives/245
All earlier ones have doubles. All newer posts have correct single titles. I have tried to edit manually, but the title shows up correctly in all places of my dashboard.
I found a suggestion on this site to check the archive.php and single.php files for specific lines of code. I am using atahualpa as my theme and do not have those files in that theme’s directory. Doing a site search only turns up those files in the twentyten theme directory and the default directory. The relevant code does not occur in the files in the default directory.
I wouldn’t doubt that there’s a code problem somewhere, since I’ve already had to fix one code problem caused by a theme update to one of my widgets. But I wouldn’t know where to begin to find said problem.